Nonproducer transformed rat cells have been derived which contain each of the three strains of defective feline sarcoma viruses. The transforming proteins encoded by these feline sarcoma viruses are expressed in these rat cells and the latter have been used to hyperimmunize their syngeneic hosts. The immune cells of these highly immunized hosts have been fused with myeloma cell lines and cloned yielding growing hybridomas which produce high-titered monoclonal antibodies to the transforming proteins. Analysis of the monoclonal antibodies by polyacrylamide gel electrophoresis revealed a battery of monospecific reactivities to unique determinants of the tranforming and structural proteins encoded by the different sarcoma viruses. Monoclonal antibody to the transforming proteins block their associated tyronsine-specific protein kinase activity indicating that this activity represents an intrinsic property of these onc gene products.